1. Field of the Invention
The present invention relates to a head drum assembly of a tape recorder such as a VCR, a camcorder and a DVC (Digital Video Camera), and more particularly, to a head drum bearing having an improved bearing preload method and structure, and a head drum assembly employing the head drum bearing and an assembling method thereof.
2. Description of Related Art
In tape recorders such as VCRs, camcorders and DVCs (Digital Video Camera), there is provided a head drum assembly which rotates at a high speed to record/reproduce information on/from a magnetic tape with the scanning operation of a magnetic head with respect to the magnetic tape. As shown in FIG. 1A, such a head drum assembly includes a rotary drum 10 rotatably supporting a magnetic head h that scans a running magnetic tape to record/reproduce information; a stationary drum 20 press-fitted to a shaft 30, which is engaged in a central axial bore of the rotary drum 10, in parallel relation with respect to the rotary drum 10; and an upper bearing and a lower bearing provided one on top of the other, and press-fitted between the rotary drum 10 and the shaft 30. Reference numerals 1 and 2 indicate a rotor and stator, respectively, and reference numeral 3 indicates a magnet which is disposed on an inner circumference of the rotor 1.
The head drum assembly of a tape recorder with the above-described construction is provided with a preload means for applying a preload to the upper and the lower bearings 40 and 50, which usually has a preloading boss 60 formed at an upper portion of the upper bearing 40 and a rib 70 formed at the stationary drum 20 under the lower bearing 50.
According to the conventional bearing preload construction, in order to apply preload to the bearings, the preloading boss 60 is secured by a setscrew s in a manner such that a protrusion of the preloading boss 60 downwardly presses an inner race 41 of the upper bearing 40, and the rib 70 upwardly presses an inner race 51 of the lower bearing 50.
As the conventional bearing preload construction of the head drum assembly additionally requires the setscrew s for securing the preloading boss 60, the manufacturing cost increases. Further, because it is not easy to determine the position of the preloading boss 60 for the securing by the setscrew s, it is difficult to control the preload, and this contributes to poor quality, such as noise generation, at the time of driving the bearings.
Furthermore, according to the conventional bearing preload method and construction of the head drum assembly, in order to remove thermal deformation caused by the excessive press-fitting of the bearing, the head drum assembly is processed by a heat treatment, such as annealing, after assembling of the bearing. Therefore, there are problems in that productivity deteriorates and manufacturing costs increase.
FIG. 1B shows residual stress being exerted in a conventional bearing assembly mounted in a head drum assembly of a tape recorder. Referring to FIG. 1B, when a lower bearing 64 is press-fitted onto a central shaft 66 in an arrowed direction 72, a force F is exerted opposite to the press-fitting direction 72 of the lower bearing 64. Accordingly, there remain the repulsive forces inside the central shaft 66 which tend to push the lower bearing 64 outward from the central shaft 66, and also the residual stress that are exerted in an arrowed direction 72 to push an inner race 67 of the lower bearing 64 downwardly. The repulsive forces 73 are particularly concentrated between the central shaft 66 and the inner race 67 of the lower bearing 64, and the residual stress 75 in an oblique direction is much more concentrated at an upper side of the lower bearing 64 than a lower side. These concentrated forces keep pushing the inner race 67 downwardly and twisting the central shaft 66, and subsequently, the initial assembling position is minutely changed due to excessive stresses caused by long use, high speed rotation, and thermal deformation caused by heat. As a result, the precise assembling of the bearing assembly cannot be guaranteed, which is a critical disadvantage especially in the field of precision instruments, which is highly sensitive to height variation of the bearing.
FIG. 1C shows another conventional head drum assembly mounted in a compact-size tape recorder such as a DVC (Digital Video Camera). For purpose of compactness, the head drum assembly of FIG. 1C has so-called direct bearings 83 and 84 which are disposed on an upper part and a lower part of a shaft 80, respectively. The direct bearings 83 and 84 include outer races 85 and 97, respectively, for supporting balls 93 and 95 seated in grooves 81 and 82 formed along an outer circumference of the shaft 80 at an upper side and a lower side.
An upper drum 90, having a plurality of magnetic heads 94, is rotatably disposed on an outer circumference of the direct bearings 83 and 84, and a lower drum 92 is press-fitted to the shaft 80 under the lower drum 90. Reference numeral 96 indicates a spring which functions as a preload means for applying a preload to the direct bearings 83 and 84, and reference numerals 96a and 96b indicate spring holders for supporting the spring 96.
Since the conventional head drum assembly with the above construction requires large grooves to directly insert the balls 93 and 95 of the upper and the lower bearings 83 and 84 in the shaft 80, the size of the shaft 80 is enlarged, especially in its diameter. Accordingly, the overall size of the head drum assembly also increases.
Also, since there is no inner race in the upper and the lower bearings 83 and 84 and the balls 93 and 95 are directly inserted in the grooves 81 and 82 of the shaft 80 to rotate the bearings, it is difficult to insert the shaft 80 into the upper and the lower bearing 83 and 84. Additionally, the head drum assembly is designed such that an outer diameter of the upper bearing 83 is larger than an inner diameter of the upper drum 90, while an outer diameter of the lower bearing is several microns smaller than a lower bearing fixing portion 90a of the upper drum 90. Therefore, the upper part of the upper bearing 83 has to be press-fitted into the upper drum 90, and interference occurs after the assembling. Since the upper bearing 83 is press-fitted into the upper drum 90 under a certain pressure, damage can occur to the upper and the lower bearings 83 and 84, and the shaft 80. As the residual stress grows in the upper and the lower bearing 83 and 84 and the shaft 80, use of the tape recorder for a long time can cause displacement of the magnetic head as a result of thermal deformation.
According to a conventional assembly method of the head drum assembly, the shaft 80 is first inserted into the upper bearing 83. The upper spring holder 96a and the spring 96 are then inserted, followed by the lower spring holder 96b and a plurality of balls 93. Next, a retainer (not shown) is assembled to maintain a desired distance between balls 93 and 95. With the assembling of a casing, assembling of the bearing assembly is completed.
Next, the bearing assembly is press-fitted into the upper drum 90. Since the lower bearing 84 is smaller than an inner diameter of the upper drum 90, the lower bearing 84 is smoothly inserted. However, since the outer diameter of the upper bearing 83 is larger than the inner diameter of the upper drum 90, when a pressure is exerted to the bearing assembly, the outer surface of the outer race 85 of the upper bearing 83 is press-fitted into the upper drum 90, sliding on an inner surface of the upper drum 90.
The magnetic head 94 is then disposed on a lower end of the upper drum 90 and rotary transformer 98 is bonded.
After that, the coils of the magnetic head 94 and the rotary transformer 98 are connected, a motor drum rotor 99 is bonded to the upper drum 90, and the shaft 80 is securely press-fitted to the lower drum 92.
Next, a cover drum 87 is securely press-fitted onto an upper end of the shaft 80.
According to the above described conventional assembly method of the head drum assembly of a magnetic recording/reproducing apparatus, the upper bearing 83, which has a relatively larger outer diameter, has to be press-fitted into the upper drum 90 of a relatively smaller inner diameter, and the lower bearing 84 is smoothly inserted into the upper drum 90 during assembling. However, damage can also occur to the bearings and the upper drum during the assembling process due to narrow gaps between the parts, and accordingly, precise assembling cannot be guaranteed. Further, the assembling process and control thereof is complicated. There is also excessive residual stress exerted to the upper drum 90. As a result, there is a disadvantage in the above described conventional assembly method of the head drum assembly of a magnetic recording/reproducing apparatus in that the assembling precision of the upper and the lower drum 90 and 92 deteriorates by age hardening.